What's the bets way to get an atmosphere effect?
It's possible to do with Volumetrics and other effects, but generally not worth it for both render time and resulting quality. Your best bet would be compositing with Z pass (Z depth) or Mist pass. It would take some tutorials for in-depth compositor nodes for making it inside Blender, but overall the idea is: you render your scene image and another pass(Z or Mist)  - a black and white image which goes from black to white depending on the distance from camera to the most distant rendered object. Then you use that pass in compositing the final image with your rendered scene, either inside Blender Compositor with nodes or image editor (like Photoshop).